It has previously been known to employ a silicone controlled rectifier (SCR) in speed control circuits for AC motors. Particularly, it has been common to use such a device in hand tools and the like where speed control is desired. A diac, interconnected to the gate of the SCR, is typically controlled by a charging circuit to provide power phase control to the motor.
The structure of the prior art control circuitry is shown in FIG. 1, where a speed control circuit is designated generally by the numeral 10. As shown, an AC power source 12 is adapted for interconnection with a motor 14 having an SCR 16 interconnected therewith. The gate of the SCR 16 is connected to a diac 18 which is then operative to gate the SCR 16 into conduction at determinable times on alternating half cycles of the AC power source 12. The time at which such conduction commences is determined by a timing circuit comprising a resistor 20, in series connection with a variable resistor 22, which is adjustable by an operator through a trigger switch or the like of the associated hand tool. As shown, the resistors 20, 22 interconnect the power source 12 with a charging capacitor 24. On alternating half cycles of the AC power source 12, the capacitor 24 charges through the resistors 20, 22 to a characteristic breakdown level of the diac 18 which then conducts to actuate the SCR 16 through its gate. The time constant of the RC circuit comprising resistors 20, 22 and capacitor 24 controls the phase angle of actuation of the SCR 16 on the conduction half cycles.
In the prior art, a clamping diode 26 has been required across the capacitor 24 for smooth control of motor operation. The diode 26 clamps the capacitor on each non-conducting half cycle to assure smooth control and low voltage start up when used with universal motors and other loads.
In the past, diacs have been found to be leaky, not given to repeatability in operation. Typically, such diacs are of an NPN construction, wherein the leakage paths in the diac are resistive paths between the layer interfaces from a surface or contact area to the base region of P material. It is most desirable in the art to eliminate the leakiness in the diacs to make the same more reliable and repeatable in operation. It is further desirable to eliminate the need for a clamping diode while still assuring smooth motor control and operation.